Two-dimensional symbols are used for most direct part marking applications because such symbols can encode a sufficient number of characters while maintaining a relatively compact size. Error correction schemes are typically included in most two dimensional symbologies, to improve the readability. The two-dimensional symbols are marked directly on the part or component using various methods, depending upon the material composition, part application, and environmental conditions. Common methods include dot peening, laser, and electro-chemical etch.
Symbols marked directly on a part or component are often difficult to read in comparison to symbols printed on labels. An image of a dot-peened, or etched mark often exhibits very low contrast using illumination or image formation systems of most image-based symbology readers. When a symbol is printed on an adhesive label, and applied to a part or component, or the part packaging, a typical image-based symbology reader can provide sufficient read rate. Industry trends, however, suggest that applications of direct part marking are nevertheless increasing.
Illumination of a symbol or mark is a primary concern when acquiring an image of the symbol to be read and decoded by an image-based symbology reader. Where symbols are printed on labels, or when marked directly on a part with a flat surface, high-angle “bright field” illumination is the type of illumination often used. Bright field illumination is a term of art for a type of illumination that produces a dark object in a bright background. Under bright field illumination, high-angle illumination strikes the object nearly perpendicularly, or at an angle not greater than 45 degrees from perpendicular. This type of illumination results in substantial illumination reflecting back toward the reader.
When a symbol or mark is etched or peened onto the surface of part or component, and the surface is rough or irregular, high-angle bright field illumination may not be appropriate. The irregular surface of the mark features will scatter as much light back to the reader as the surface of the background, resulting in indistinguishable features in the image. Low angle, “dark field” illumination has been shown to be suitable for certain direct part marking applications.
Under dark field illumination, low-angle illumination strikes the object at a low angle from the surface of the object, i.e., at an angle between 45 degrees and 90 degrees from perpendicular. Dark field illumination reflects away from the reader, with only random, irregular features of the mark or symbol on the surface of the object reflecting a portion of the illumination back into the reader. Further, certain reading applications may yield higher successful read rates when a combination of bright field and dark field illumination is used.
Industrial symbology readers are typically designed to provide bright field illumination from lighting sources integrated into the reader. When these readers are adapted to read direct part mark symbols, the use of removable light pipes and other adapters are used to redirect bright field illumination into a dark field mode. Accordingly, hand held readers known in the art that are used for direct part mark reading applications are configured exclusively for either bright field mode or dark field mode.
Accordingly, there is a need for a direct part mark reader with integrated illumination that provides variable and controllable illumination in both bright field and dark field modes.